My gut is to say that any time there seems to be easy money to be made, the opportunity would fade as everyone jumped on it.
Let me ask you - why do you think these stocks are priced to yield 7-9%? The DVY yields 3.41% as of Aug 30,'12.
The high yielding stocks you discovered may very well be hidden gems. Or they may need to reduce their dividends and subsequently drop in price.
No, it's not 'safe.' If the stocks you choose drop by 20%, you'd lose 40% of your money, if you made the purchase on 50% margin. There's risk with any stock purchase, one can claim no stock is safe. Either way, your proposal juices the effect to creating twice the risk.
Edit - After the conversation with Victor, let me add these thoughts. The "Risk-Free" rate is generally defined to be the 1yr tbill (and of course the risk of Gov default is not zero). There's the S&P 500 index which has a beta of 1 and is generally viewed as a decent index for comparison. You propose to use margin, so your risk, if done with an S&P index is twice that of the 1X S&P investor. However, you won't buy S&P but stocks with such a high yield I question their safety. You don't mention the stocks, so I can't quantify my answer, but it's t-bill, The S&P, Twice the S&P, then you.